Dylann Roof, 22, is charged in the June 2015 deaths of nine black parishioners at Emanuel AME Church

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In this June 18, 2015, file photo, Charleston, S.C., shooting suspect Dylann Storm Roof, center, is escorted from the Sheby Police Department in Shelby, N.C. Joey Meek, a friend of Roof's, told an FBI agent that he did not know specifics about Dylann's plan to shoot the churchgoers during Bible study, but the FBI says that was a lie.

A white man charged with the shooting deaths of nine black churchgoers in Charleston "self-radicalized" in the months before the attack and grew more entrenched in his beliefs in white supremacy, according to court papers prosecutors filed this week in federal court.

The information filed Monday was part of a list of more than a dozen expert witnesses that prosecutors intend to call in Dylann Roof's federal death penalty trial later this year.

Roof, 22, is charged in the June 2015 deaths of nine black parishioners at Emanuel AME Church. He also faces a possible death sentence if convicted of murder charges in state court.

Roof's actions, according to the government, were "consistent with the concept of leaderless resistance and martyrdom advocated by white supremacy extremist groups and self-radicalization leading to violence." Experts in white supremacy, the government notes, are expected to testify on Roof's "extremist ideology, including a belief in the need to use violence to achieve white supremacy."

Roof's increasing tendency toward white supremacist ideology, the government alleges, came in Roof's "travel to such race-relevant destinations as the site of his crimes and locations that have connections to the antebellum and Confederate eras." Experts will also comment on patterns in Roof's travel, personal interests and dress, which the government says will show as being "consistent with the adoption of white supremacist beliefs through self-radicalization."

Authorities have long said they believed Roof was the author of an online manifesto in which he embraced Confederate symbols and talked about white supremacy. Handwriting analysis experts are expected to testify that he wrote another similar document while jailed after his arrest.

Also Monday, a media attorney objected to closing a hearing on request by Roof's defense to keep some evidence out of Roof's federal trial. U.S. District Judge Richard Gergel has set a Sept. 1 hearing, which he has said he intends to close to protect Roof's right to a fair trial.

Attorney Jay Bender, representing The Post and Courier of Charleston, has written the judge asking to be heard before any decision to close the hearing. Bender says reasonable alternatives should be considered and if the hearing is closed, reasons for doing so should be on the record.

In another filing Monday, prosecutors disputed earlier arguments made by Roof's attorneys that the death penalty is unconstitutional and should therefore be removed as a possible punishment if he's convicted in federal court.